November 14, 2018, 4:17 pm

Using Performance Measures to Drive Maintenance and Asset Management Performance Improvement


Element 7 – Be aware of the “natural” variability of any process

Of course, most engineers understand that any process has a certain degree of natural variability.  But it is absolutely essential that this variability is understood when analysing performance data, and before taking appropriate action.  Imagine, for example, the following performance data as shown in Figure 3.


Tonnes per hour graph 1

Figure 3

Clearly, performance is dropping, and a manager may decide to take some action, usually by making his displeasure with the situation known.  Following this action, performance data continues for the next two weeks as shown in Figure 4.

    Tonnes per hour graph2 

Figure 5

Tonnes per hour graph3 

Figure 4

Clearly (in his view), his action has had the desired results, as the plant throughput has increased significantly.  Obviously, “kicking butt” when performance is low is an effective strategy to turn performance around.  Having had the desired effect, the manager may then choose (in a temporary fit of remorse) to reward his people, and let them know that he is happy with the improved performance.  After this act of generosity, performance trends as shown in Figure 5.

Despite the reward that he has given his people, performance has dropped.  Clearly, this reinforces in the manager’s mind that rewarding people for good performance is a counter-productive strategy, as all that it leads to is lower performance.  But what if we overlay on this chart the long run confidence limits for this process, based on statistical analysis of the variability of past performance – the limits within which we can expect that 95% of results will lie.  This represents the natural variability of the process – any variation within these limits is due to the process itself, rather than any external intervention.  This is shown in Figure 6.

 Figure 6

Figure 6

From this chart, we can see that all the data points lie within the Upper and Lower Control Limits for the process.  In other words, despite the impression that the manager had that his actions were influencing the results, the reality is that all changes were as a result of the natural variability of the process, rather than his actions. 

Furthermore, none of the results on the chart indicate that the process is “out of control”, and therefore requires some action or intervention.  So the manager has actually been wasting his time focusing on results that do not require his attention.

So you can see that understanding the natural variability of performance is important in allowing us to focus on the things that deserve our attention.

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